1. Field of the Invention
The present invention relates to magnetic memory elements, and more specifically to magnetic tunnel junctions having improved synthetic anti-ferromagnetic structures.
2. Brief Description of the Related Art
Various types of memory are used in digital systems such as microprocessor-based systems, digital processing systems, and the like. Recently, magnetic random access memory (MRAM) devices have been developed for use as non-volatile random access memory.
MRAM devices are based on magnetic memory elements. An MRAM device frequently includes several magnetic memory elements arranged in an array of rows and columns, with circuitry for accessing information stored in individual elements in the array.
Information is stored in each magnetic memory element as a resistance state of the element. The elements typically are layered structures, and an electrical resistance state of each element changes based on the relative orientations of magnetic moments in ferromagnetic layers within the element. The orientation of the magnetic moment in one layered structure, referred to as a “pinned” structure, is fixed as a reference, while the magnetic moment orientation of another layered structure, referred to as a “free” or “sense” structure, can be changed. The magnetic moment orientation of the free structure can be changed using an externally-applied magnetic field or current, for example. Changing the relative magnetic orientation of the two layered structures results in a change in the resistive state of the magnetic memory element. The different resistive states are recognized by electronic circuitry as bit-wise storage of data.
Typically, the free and pinned ferromagnetic structures in a magnetic memory element are separated by a non-magnetic spacer. In one type of magnetic element, known as a magnetic tunnel junction (MTJ), the spacer is referred to as a tunnel junction barrier. When the magnetic moments of the free structure and the pinned structure are aligned in the same direction, the orientation commonly is referred to as “parallel.” When the two structures have opposite magnetic alignment, the orientation is termed “antiparallel.”
The tunnel junction barrier is sufficiently thin that, in the presence of adequate current, quantum-mechanical tunneling of charge carriers occurs across the barrier junction between the free and pinned ferromagnetic structures. The tunneling magnetoresistance of the device typically has minimum and maximum values corresponding respectively to parallel and antiparallel magnetization moment orientations of the free and pinned structures.
In response to parallel and antiparallel magnetic states, MTJ elements present a resistance, known as the “tunneling magnetoresistance” (TMR), to a current provided across the element. In magnetic memory elements, the current typically is provided in a direction perpendicular to the surfaces of the element layers.
Current miniaturization trends in electronic componentry require that magnetic memory elements be manufactured with layers that are very thin, some layers being in the range of only tens of angstroms in thickness. At these small dimensions, minute variations in surface morphology, roughness, and constituent grain size can impact the magnetic characteristics of each layer.
Improvements in the arrangement and composition of the various layers in the magnetic memory element are desired to reduce pinning field dispersion and improve operations of magnetic memory elements based on MTJs.